In fact Storm Shadow has a much more sophisticated warhead and using it outside of its specialised role (penetrating a building like seen here (even much more fortified ones), then detonating deep inside) because you don't have much with that range is an actual waste. But that's where ATACMs fit in perfectly.

ATACMs is more a generalized and weaker missile with a few warhead variants (mostly bomblets or unitary, fragmenting on explosion) mostly used against at best medium armored targets while doing a lot of area damage. But then it can be fired more easily from all the existing platforms using GLMRS.

Storm Shadow on the other hand is a completely difference beast. A warhead twice the size and specialised to breach very hard targets including buried bunkers, where you can even pre-program how deep it penetrates before the main charge goes off. So when you really want to hit what's deep inside a building quite precisely, this is the right tool.